Automated lateral control of seismic streamers

ABSTRACT

In the field of marine geophysical surveying, systems and methods for controlling the spatial distribution or orientation of a geophysical sensor streamer or an array of geophysical sensor streamers towed behind a survey vessel are provided. Various techniques for changing the spatial distribution or orientation of such geophysical sensor streamers in response to changing conditions are provided. For example, crosscurrent conditions may be determined based on configuration data received from positioning devices along the length of a streamer, and a new desired orientation for the streamer may be determined based on the crosscurrent conditions. The new desired orientation may include a new desired feather angle for the streamer.

BACKGROUND

1. Technical Field

The disclosure relates generally to the field of marine geophysicalsurveying. More particularly, the disclosure relates to systems andmethods for controlling the spatial distribution or orientation ofgeophysical sensor streamer or an array of geophysical sensor streamerstowed behind a survey vessel.

2. Description of the Related Art

Marine geophysical surveying systems such as seismic acquisition systemsand electromagnetic survey systems are used to acquire geophysical datafrom formations disposed below the bottom of a body of water, such as alake or the ocean. Marine seismic surveying systems, for example,typically include a seismic survey vessel having onboard navigation,seismic energy source control, and geophysical data recording equipment.The seismic survey vessel is typically configured to tow one or more(typically a plurality) laterally spaced sensor streamers through thewater. At selected times, the seismic energy source control equipmentcauses one or more seismic energy sources (which may be towed in thewater by the seismic vessel or by another vessel) to actuate. Signalsgenerated by various sensors on the one or more streamers in response todetected seismic energy are ultimately conducted to the recordingequipment. A record is made in the recording system of the signalsgenerated by each sensor (or groups of such sensors). The recordedsignals are later interpreted to infer the structure and composition ofthe formations below the bottom of the body of water. Correspondingcomponents for inducing electromagnetic fields and detectingelectromagnetic phenomena originating in the subsurface in response tosuch imparted fields may also be used in marine electromagneticgeophysical survey systems.

The one or more sensor streamers are in the most general sense longcables that have geophysical sensors disposed at spaced-apart positionsalong the length of the cables. A typical streamer may extend behind thegeophysical survey vessel for several kilometers. Because of the greatlength of the typical streamer, the streamer may not travel entirely ina straight line behind the survey vessel at every point along its lengthdue to interaction of the streamer with the water, among other factors.

Streamers towed by a vessel configured for towing multiple streamers aregenerally associated with equipment that maintains the forward ends ofthe streamers at selected lateral distances from each other and from thecenterline of the survey vessel as they are towed through the water.Single streamers are generally used in what are known as two-dimensionalgeophysical surveys, and multiple streamer systems are used in what areknown as three-dimensional and four-dimensional surveys. Afour-dimensional seismic survey is a three dimensional survey over aparticular area of the Earth's subsurface repeated at selected times.The individual streamers in such systems are generally affected by thesame forces that affect a single streamer.

The quality of geophysical images of the Earth's subsurface producedfrom three-dimensional surveys is affected by how well the positions ofthe individual sensors on the streamers are controlled. The quality ofimages generated from the detected signals also depends to an extent onthe relative positions of the sensors being maintained throughout thegeophysical survey.

Various embodiments of streamer control systems and methods aredisclosed in U.S. Patent Publication 2012/0002502, entitled “METHODS FORGATHERING MARINE GEOPHYSICAL DATA,” which is incorporated by referenceherein.

SUMMARY

A method according to one aspect of this disclosure includes towing astreamer behind a vessel in a body of water. Information is receivedrelating to crosscurrents in the body of water, and a desiredorientation for the streamer is determined based on that information.The orientation of the streamer is then adjusted in accordance with thedesired orientation.

A method according to another aspect of this disclosure includes towinga streamer having deflecting devices arranged therealong in a body ofwater. The streamer is towed with a present streamer feather anglemeasured relative to some reference axis. The method includes receivinginformation regarding forces exerted by the deflecting devices andautomatically determining a desired streamer feather angle based on thereceived information. The method further includes automaticallyadjusting the streamer, via the deflecting devices, to follow thedesired streamer feather angle.

A streamer control apparatus according to one aspect of this disclosureincludes at least one processor configured to communicate withpositioning devices arranged along a streamer towed behind a vessel in abody of water. The processor is further configured to determineconfiguration data corresponding to the positioning devices, theconfiguration data being indicative of crosscurrent conditions. Theprocessor is further configured to adjust the positioning devices basedon the crosscurrent conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a vessel towing an array of seismic streamers includingdevices for adjusting the geometry of the respective streamers.

FIG. 2 depicts a streamer deflecting device.

FIG. 3 depicts a vessel and some possible reference axes relative towhich a streamer feather angle might be measured.

FIG. 4A depicts a vessel towing a plurality of streamers at a featherangle.

FIG. 4B depicts the vessel of FIG. 4A towing the streamers at adifferent feather angle.

FIG. 5A depicts a vessel towing a plurality of streamers in oneorientation.

FIG. 5B depicts the vessel of FIG. 5A towing the streamers in adifferent orientation.

FIGS. 6 and 7 depict two exemplary process flows according toembodiments of the present disclosure.

DETAILED DESCRIPTION

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

TERMINOLOGY

The following paragraphs provide definitions and/or context for termsfound in this disclosure (including the appended claims):

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based only in parton those factors. Consider the phrase “determine A based on B.” Thisphrase connotes that B is a factor that affects the determination of A,but does not foreclose the determination of A from also being based onC. In other instances, A may be determined based solely on B.

“Configured To.” As used herein, this term means that a particular pieceof hardware or software is arranged to perform a particular task ortasks when operated. Thus, a system that is “configured to” perform taskA means that the system may include hardware and/or software that,during operation of the system, performs or can be used to perform taskA. (As such, a system can be “configured to” perform task A even if thesystem is not currently operating).

“Orientation.” As used herein, this term includes any informationregarding the geometric arrangement of a streamer. As non-limitingexamples, the term “orientation” may include the feather angle of astreamer relative to some reference axis, the shape of a streamer, theposition of a streamer relative to another streamer, or the position ofa portion of a streamer.

“Feather angle.” As used herein, this term refers to the angle that astreamer makes relative to some reference axis. Because a streamer maynot always be arranged along a perfectly straight line, this term shouldbe interpreted to encompass any suitable way of defining an average orapproximate angle for such streamers. As non-limiting examples of suchmethods, the “approximate direction” for a streamer might be defined asthe line connecting one end of the streamer to the other end;alternatively, the approximate direction might be defined as a line ofbest fit, which might be calculated discretely or continuously invarious ways.

“Desired feather angle.” As used herein, this term refers to the featherangle that an operator or control system attempts to cause a streamer tomake relative to a reference axis. Typically, but not always, a “desiredfeather angle” will be a streamer orientation that is along a straightline. As above, however, this term should also be interpreted toencompass any suitable way of defining an average or approximate desiredangle for such streamers.

“Automatic.” As used herein, this term includes anything done by ahardware or software control device.

“Coupled.” As used herein, this term includes a connection betweencomponents, whether direct or indirect.

“Lateral control device.” As used herein, this term includes variousdevices for positioning streamers laterally. In this disclosure, suchdevices may be referred to variously as “lateral control devices,”“lateral force control devices,” “birds,” “positioning devices,”“lateral positioning devices,” and “deflecting devices.” These termsshould also be understood to encompass devices providing additionalcapabilities, such as depth control; for example, terms such as “lateralforce and depth control devices,” “LFDs,” and the like may also be usedto refer to such devices.

FIG. 1 shows a typical marine geophysical survey system that may includea plurality of sensor streamers. Each of the sensor streamers may beguided through the water by one or more lateral control devices coupledto each of the streamers. The geophysical survey system includes surveyvessel 10 that moves along the surface of body of water 11 such as alake or the ocean. Survey vessel 10 may include thereon equipment, showngenerally at 12 and for convenience collectively referred to as a“recording system.” Recording system 12 typically includes devices suchas a data recording unit (not shown separately) for making a record withrespect to time of signals generated by various sensors in theacquisition system. Recording system 12 also typically includesnavigation equipment (not shown separately) to determine and record, atselected times, the geodetic position of survey vessel 10, and, usingother devices to be explained below, each of a plurality of geophysicalsensors 22 disposed at spaced-apart locations on streamers 20 towed bysurvey vessel 10.

In one example, the device for determining the geodetic position may begeodetic position signal receiver 12A such as a global positioningsystem (“GPS”) receiver, shown schematically at 12A. Other geodeticposition determination devices are known in the art, such as otherglobal navigation satellite systems. The foregoing elements of recordingsystem 12 are familiar to those skilled in the art, and with theexception of geodetic position detecting receiver 12A, are not shownseparately in the figures herein for clarity of the illustration.

Geophysical sensors 22 may be any type of geophysical sensor known inthe art. Non-limiting examples of such sensors may includeparticle-motion-responsive seismic sensors such as geophones andaccelerometers, pressure-responsive seismic sensors,pressure-time-gradient-responsive seismic sensors, electrodes,magnetometers, temperature sensors or combinations of the foregoing. Invarious implementations of the disclosure, geophysical sensors 22 maymeasure, for example, seismic or electromagnetic field energy primarilyreflected from or refracted by various structures in the Earth'ssubsurface below the bottom of body of water 11 in response to energyimparted into the subsurface by energy source 17. Seismic energy, forexample, may originate from a seismic energy source, or an array of suchsources, deployed in body of water 11 and towed by survey vessel 10 orby another vessel (not shown). Electromagnetic energy may be provided bypassing electric current through a wire loop or electrode pair (notshown for clarity). The energy source (not shown) may be towed in bodyof water 11 by survey vessel 10 or a different vessel (not shown).Recording system 12 may also include energy source control equipment(not shown separately) for selectively operating energy source 17.

In the survey system shown in FIG. 1, there are four sensor streamers 20towed by survey vessel 10. The number of sensor streamers shown in FIG.1, however, is only for purposes of illustration and is not a limitationon the number of streamers that may be used in any particularembodiment. As explained in the Background section herein, in marinegeophysical acquisition systems such as shown in FIG. 1 that include aplurality of laterally spaced-apart streamers, streamers 20 aretypically coupled to towing equipment that secures the forward end ofeach of streamers 20 at a selected lateral position with respect toadjacent streamers and with respect to survey vessel 10. As shown inFIG. 1, the towing equipment may include two paravanes 14 coupled tosurvey vessel 10 via paravane tow ropes 8. Paravanes 14 are theoutermost components in the streamer spread and are used to providestreamer separation.

Paravane tow ropes 8 are each coupled to survey vessel 10 at one endthrough winch 19 or a similar spooling device that enables changing thedeployed length of each paravane tow rope 8. In the embodiment shown,the distal end of each paravane tow rope 8 is coupled to paravanes 14.Paravanes 14 are each shaped to provide a lateral component of motion tothe various towing components deployed in body of water 11 whenparavanes 14 are moved therethrough. The lateral motion component ofeach paravane 14 is opposed to that of the other paravane 14. Thecombined lateral motion component of paravanes 14 separates paravanes 14from each other until they put into tension one or more spreader ropesor cables 24, coupled end to end between paravanes 14.

Streamers 20 may each be coupled, at the axial end thereof nearestsurvey vessel 10 (the “forward end”), to respective lead-in cableterminations 20A. Lead-in cable terminations 20A may be coupled to orassociated with spreader ropes or cables 24 so as to fix the lateralpositions of streamers 20 with respect to each other and with respect tothe centerline of survey vessel 10. Electrical, optical, and/or anyother suitable connection between the appropriate components inrecording system 12 and, ultimately, geophysical sensors 22 (and/orother circuitry) in the ones of streamers 20 inward of the lateral edgesof the system may be made using inner lead-in cables 18, each of whichterminates in respective lead-in cable termination 20A. Lead-intermination 20A is disposed at the forward end of each streamer 20.Corresponding electrical, optical, and/or other suitable connectionbetween the appropriate components of recording system 12 andgeophysical sensors 22 in the laterally outermost streamers 20 may bemade through respective lead-in terminations 20A, using outermostlead-in cables 16. Each of innermost lead-in cables 18 and outermostlead-in cables 16 may be deployed by respective winches 19 or similarspooling devices such that the deployed length of each cable 16, 18 maybe changed. The type of towing equipment coupled to the forward end ofeach streamer shown in FIG. 1 is only intended to illustrate a type ofequipment that can tow an array of laterally spaced-apart streamers inthe water. Other towing structures may be used in other examples ofgeophysical acquisition system according to the present disclosure.

The acquisition system shown in FIG. 1 may also include a plurality oflateral control devices 26 coupled to each streamer 20 at selectedpositions along each streamer 20. Each lateral control device 26 mayinclude one or more rotatable control surfaces (not shown separately inFIG. 1; see FIG. 2 for an exemplary embodiment) that when moved to aselected rotary orientation with respect to the direction of movement ofsuch surfaces through the water 11 creates a hydrodynamic lift in aselected direction to urge streamer 20 in a selected direction. Thus,such lateral control devices 26 may be used to maintain streamers 20 ina selected orientation. The particular design of the lateral controldevices 26, however, is not a limit on the scope of the presentdisclosure.

In one embodiment, position determination devices may be associated withlateral control devices 26. In one example, the position determinationdevice may be an acoustic range sensing device (“ARD”) 26A. Such ARDstypically include an ultrasonic transceiver or transmitter andelectronic circuitry configured to cause the transceiver to emit pulsesof acoustic energy. Travel time of the acoustic energy between atransmitter and a receiver disposed at a spaced-apart position such asalong the same streamer and/or on a different streamer, is related tothe distance between the transmitter and a receiver, and the acousticvelocity of the water. The acoustic velocity may be assumed not tochange substantially during a survey, or it may be measured by a devicesuch as a water velocity test cell. Alternatively or additionally, ARDsmay be disposed at selected positions along each one of the streamersnot co-located with the lateral control devices 26. Such ARDs are shownat 23 in FIG. 1. Each ARD 26A, 23 may be in signal communication withrecording system 12 such that at any moment in time the distance betweenany two ARDs 26A, 23 on any streamer 20 is determinable. One or moreARDs may be placed at selected positions proximate the rear end ofsurvey vessel 10 so that relative distances between the selectedpositions on survey vessel 10 and any of the ARDs on the streamers mayalso be determined.

Streamers 20 may additionally or alternatively include a plurality ofheading sensors 29 disposed at spaced-apart positions along eachstreamer 20. Heading sensors 29 may be geomagnetic direction sensorssuch as magnetic compass devices affixed to the exterior of streamer 20.Heading sensors 29 provide a signal indicative of the heading (directionwith respect to magnetic north) of streamer 20 at the axial position ofheading sensor 29 along the respective streamer. Measurements of suchheading at spaced-apart locations along each streamer may be used tointerpolate the orientation (including the spatial distribution) of eachstreamer.

Each streamer 20 may include at the distal end thereof a tail buoy 25.Tail buoy 25 may include, among other sensing devices, geodetic positionreceiver 25A such as a GPS receiver that may determine the geodeticposition of each tail buoy 25. The geodetic position receiver 25A ineach tail buoy 25 may be in signal communication with recording system12.

By determining the distance between ARDs 26A, 23, including the one ormore ARDs on survey vessel 10, and/or by interpolating the spatialdistribution of the streamers from heading sensor 29 measurements, anestimate of the orientation of each streamer 20 may be made.Collectively, the orientation of streamers 20 may be referred to as the“array orientation.”

The various position measurement components described above, includingthose from heading sensors 29, from ARDs 26A, 23, and, if used, fromadditional geodetic position receivers 25A in tail buoys 25, may be usedindividually or in any combination. The ARDs and heading sensors may bereferred to for convenience as “relative position determination”sensors. By determining relative positions at each point along eachstreamer with reference to a selected point on the survey vessel or theenergy source, is it possible to determine the geodetic position of eachsuch streamer point if the geodetic position of the vessel or the energysource is determined. As explained above, the navigation portion ofrecording system 12 may include a GPS receiver or any other geodeticlocation receiver 12A. In some examples, energy source 17 may alsoinclude a geodetic position location receiver 17A such as a GPSreceiver.

During operation of the geophysical acquisition system shown in FIG. 1,it may be desirable to adjust portions of the streamers 20 laterally inorder to maintain a desired streamer orientation or array orientationduring geophysical surveying. Recording system 12 may be configured tosend suitable control signals to each lateral control device 26 to moveassociated portions of each streamer 20 laterally. Such lateral motionmay be selected so that each point along each streamer is located at apredetermined relative position at any moment in time. The relativepositions may be referenced to the position of either survey vessel 10or energy source 17. Examples of various array orientation control modesaccording to this disclosure are provided below.

During operation of the acquisition system shown in FIG. 1 when used forseismic surveying, for example, it may be desirable for streamers 20 tobe arranged as evenly as practicable behind survey vessel 10 to avoidholes in the survey coverage. “Evenly” or “even” in the present contextmeans that it is desirable that streamers 20 are parallel to each otheralong their length, that there is equal lateral distance betweenadjacent streamers, and that the streamers extend parallel to a selecteddirection. Deviation from such an even arrangement may be caused by ripcurrents, crosscurrents, and propeller wash from survey vessel 10, amongother causes. Holes in the coverage is a condition wherein seismicsensors are disposed more sparsely than would be the case if theorientation of the array were even, as defined above.

For purposes of this disclosure, the term “parallel” may be defined interms of the “approximate directions” of streamers, as were discussedabove. One of ordinary skill in the art will recognize that differentlevels of parallelism may be sufficient for different purposes. Forexample, in various embodiments, two streamers may be considered“parallel” if their approximate directions differ by at most 0.1°, 0.5°,1°, 2°, 3°, 4°, 5°, 10°, 15°, or 20°. For purposes of this disclosure,“parallel” may be taken to mean “having approximate directions within5°,” and “substantially parallel” may be taken to mean “havingapproximate directions within 10°.”

FIG. 2 shows an example of a bird 30 capable of providing lateralcontrol to a streamer.

Bird 30 includes attachment devices 32 for being coupled to a streamer20. As streamer 20 and bird 30 move through the water, the angle of wing34 about wing axis 33 determines the amount of lateral force provided bybird 30 to streamer 20. This wing angle may be controlled at a pluralityof birds 30 attached to streamer 20 to provide the desired amount anddirection of lateral force at various points along the length ofstreamer 20, in order to change the orientation of the streamer. Manydifferent types of lateral control devices are known in the art, andbird 30 is provided only as an example of such a device.

FIG. 3 shows exemplary reference axes relative to which a streamerfeather angle may be measured. The heading of survey vessel 10 is oneviable choice, shown as heading axis 50. In the figures that follow,heading axis 50 will be used; other options, as discussed below, arealso possible.

In the presence of crosscurrent 52, the actual direction of travel ofsurvey vessel 10 may differ from its heading; thus direction of travelmay also be a useful reference axis. This is shown as direction oftravel axis 54. Other possibilities include true north 56 and magneticnorth 58. Other possibilities (not shown) include the streamer-front-enddirection and the preplot direction for the survey. What is meant by“preplot direction” is the ideal track of the vessel. For example, in a3D survey, the preplot lines are typically equally distributed, parallel(or substantially parallel) lines along the survey area, separated by adistance equal to the width of the area covered in one pass. In a 4Dsurvey, the preplot direction typically follows the actual previoustrack of the vessel. The preplot direction thus may be constant (such asfor each line of a 3D survey) or variable (such as in 4D surveys).

In some embodiments, the preplot lines may be circular. For example, theideal track of the vessel may be a series of overlapping, continuouslylinked circles. The circles may have approximately the same focus ordifferent foci. In these embodiments, the paths of streamers 20 areequally distributed over a predetermined area around the preplot line.For example, streamers 20 can be equally distributed across apredetermined lateral width.

Some of the more common choices for a reference axis have been provided;however, a reference axis may be any suitable axis and merely provides aframe of reference for measuring streamer feather angles.

FIG. 4A shows survey vessel 10 towing a plurality of streamers 20. Forsimplicity, paravanes 14, paravane tow ropes 8, lead-in cables 16 and18, lead-in cable terminations 20A, and spreader ropes or cables 24 arenot shown separately in this figure or the figures that follow. Thesevarious components have been combined into rigging 64. Further, thevarious devices along the length of streamers 20 have been omitted forsimplicity.

As shown, heading axis 50 has been chosen as the reference axis in thisexample. Streamers 20 are shown oriented at initial feather angle 62relative to heading axis 50. Control equipment (not shown) may beconfigured to control the birds arranged along each streamer 20 toprovide the necessary wing angles to maintain a particular featherangle. It is typically desirable to have streamers at a feather anglenear zero (relative to either heading axis 50 or direction of travelaxis 54). With crosscurrents, however, a feather angle of zero may notalways be feasible. Further, crosscurrents may vary, both as a functionof time as the survey progresses, and as a function of position alongthe length of streamers 20. Thus in some instances, a bird mayexperience a particularly strong crosscurrent that must be counteractedto maintain the feather angle. Accordingly, some birds may have to useexcessive wing angles to provide the required amount of force tomaintain a particular feather angle and/or streamer orientation.Increasing the amount of force produced by a bird tends to increase theturbulence and noise generated thereby, which may negatively impact thequality of the data gathered in the survey. Accordingly, in somesituations, it may be desirable to decrease the noise generated by thebirds by changing the feather angle. In one embodiment, this change mayinvolve increasing the feather angle.

According to one embodiment of the present disclosure, changing thefeather angle may be carried out by attempting to determine a featherangle that reduces the sum of the forces generated by the birds (or byany chosen subset of the birds, or the force generated by a particularbird). In some embodiments, a feather angle may be determined to attemptto minimize or significantly reduce such forces. Because minimizationmay not always be possible or feasible, approximate minimization may bean acceptable alternative to minimization. Various levels of approximateminimization may be considered sufficient in various embodiments. Forexample, the maximum force that can be applied by a bird may be taken tobe 100%, and the actual minimum force possible may be taken to be 0%.For purposes of this disclosure, however, the term “minimization” shouldbe interpreted to include anything less than or equal to 5% of themaximum force. The term “approximate minimization” should be interpretedto include anything less than or equal to 20% of the maximum force. Invarious other embodiments, it may be considered sufficient for the valueto be anything less than or equal to 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%,10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50% of the maximum force.

In this way, a desired feather angle reducing, minimizing, orapproximately minimizing a desired force or a desired force sum may bedetermined. This desired feather angle will tend to simply follow theaverage crosscurrent. This embodiment may be useful in situations wherethe actual feather angle is not of particular importance, but minimizingbird noise is important. Bird noise may typically be a larger concernthan feather angle, one of ordinary skill in the art will understand thetradeoff between steering and acceptable noise level. Further, in somecases it may not be possible or feasible to reach a desired featherangle due to strong crosscurrents; in such cases, it may be advantageousto use a strategy that allows the feather angle to follow the averagecrosscurrent, keeping the streamers straight and correctly separated.

FIG. 4B shows the same elements as FIG. 4A, but feather angle 62 hasbeen adjusted to new feather angle 72, which is increased relative tofeather angle 62. The actual angles depicted are not necessarily toscale. The increase from feather angle 62 to new feather angle 72 istypically carried out in order to reduce the forces generated by thebirds. However, the pursuit of various objectives may lead to differentvalues for new feather angle 72.

As described above, the increase in feather angle typically leads to areduction in bird forces. This force reduction may be carried out onetime, continuously, or periodically based on the bird forces. In oneembodiment, the force reduction may be based on the instantaneous (e.g.,momentary) forces produced by the birds. Additionally, force reductionmay include a time-filtering, time-averaging, and/or force integrationaspect in order to suppress any potential instability and/oroscillations in the determined desired feather angle that may be causedby adjustments to the feature angle every second where short-duration(e.g., on the scale of one-second) changes in crosscurrents may have anundesirably large impact on the desired feather angle. For example, theforces being produced by the birds may be averaged over a 30-second timeinterval, a 60-second time interval, a two-minute time interval, or anyother suitable interval, to determine a desired feather angle that isless dependent upon momentary fluctuations in crosscurrents and birdforces.

A desired feather angle may be determined not simply to reduce,minimize, or approximately minimize bird forces, but to maintain thembelow some desired threshold value while keeping the desired featherangle as close as possible to some reference feather angle (e.g., apredefined ideal value). In this embodiment, the feather angleadjustment also may be carried out one time, continuously, orperiodically based on the bird forces. The feather angle may also bebased on the instantaneous (e.g. momentary) forces produced by thebirds; however, the feather angle may include a time-filtering,time-averaging, and/or force integration aspect in order to suppressinstability and/or oscillations in the determined desired feather angle.This embodiment may be useful in situations where a tradeoff between anoptimal feather angle and noise produced by bird forces is desired.

These embodiments of reducing bird forces or maintaining bird forcesbelow a threshold may also depend on certain other conditions. Forexample, the control system might require operator confirmation beforeimplementing a feather angle change. In some embodiments, the controlsystem may allow feather angle changes only at the end of a survey lineand before the next survey line begins, in order to provide a consistentfeather angle for each survey line.

FIGS. 5A and 5B show another embodiment of adapting streamer orientationin response to forces being produced by the birds. In FIG. 5A, surveyvessel 10 is towing streamers 20 at an initial feather angle ofapproximately zero, relative to heading axis 50. This configuration maybe desirable when crosscurrents are relatively small to give good surveycoverage.

As noted above, however, crosscurrents may vary not just with time, butalso along the length of streamers 20. As shown in FIG. 5B, a strongcrosscurrent at the forward portion of streamers 20 has deflected theforward portion of streamers 20 to some extent. However, survey vessel10 has not traveled far enough for the rear portion of streamers 20 tohave encountered this crosscurrent yet. In a situation such as this, itmay be desirable to adapt the streamer feather angle to take intoaccount the fact that the rear portions of streamers 20 are likely toexperience a similar crosscurrent, but at a later time. Thus, a desiredfeather angle may be chosen to proactively position the rear portions ofstreamers 20 along a feather angle that takes into account this strongcrosscurrent. Accordingly, the control system has determined new desiredfeather angle 82.

Bird forces along the length of streamers 20 may then be determined inorder to identify a desired orientation for the streamer in a straightline at a new desired feather angle 82. It is appreciated that while aperfectly straight line may be the optimal arrangement for a streamer,no such perfectly straight line exists in nature. Accordingly, minordeviations from perfection may be considered acceptable here. It isfurther appreciated that while the desired orientation may in fact be aperfectly straight line, in the real world the actual orientation willalways be an approximation thereof.

By way of example, one method of quantifying the “straightness” of astreamer might be as follows. Let “T_(actual)” be defined as thedistance between the two ends of the streamer as they are positioned inthe water (i.e. the length of the streamer in its actual configuration).Let “L_(straight)” be defined as the length the streamer would have ifit were completely straight (i.e. the ideal length of the streamer). Thestreamer's straightness “S” can then be defined asS=L_(actual)/L_(straight). Under this definition, S can be seen as apercentage value that indicates how “close” to being straight thestreamer is. In various embodiments, it might be sufficient for astreamer to have an S value of 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%,90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, or 100%.

For purposes of this disclosure, the term “straight” may be taken tomean an S value of 90% or more. The term “approximately straight” may betaken to mean an S value of 80% or more.

Bird forces 84, 86, 88, 90, and 92 are shown as distinct in FIG. 5B inorder to illustrate that various forces may be desired at differentpositions along the length of streamers 20, and they need not be thesame as one another. By using the predictive aspect of this embodiment,the peak forces required from the birds may be reduced.

FIG. 5B illustrates the above-described situation by showing the casewhere the crosscurrent is strong enough that the birds attached to theforward portion of streamers 20 are unable to maintain the feather angleof zero (or whatever the initial feather angle may have been). A similarsituation may occur, however, when the birds attached to the forwardportion of streamers 20 are able to maintain the initial feather angle,but only by producing undesirably large lateral forces. In thatsituation as well, it may be desirable for the control system todetermine new desired feather angle 82.

It should be noted that, while the above discussion focuses on the useof forces produced by birds in determining a desired streamer featherangle or orientation, various other quantities may also be used as asubstitute for force. For example, configuration data for the birds maybe a useful proxy for force. Configuration data may include informationregarding the wing angles of birds, or other information indicative ofthe birds' current state or indicative of how much force or noise thebirds are outputting to maintain a feather angle and/or streamerorientation. Further, configuration data may include any informationindicative of crosscurrents, including but not limited to directmeasurement of crosscurrents.

FIG. 6 shows an exemplary process flow for an embodiment according tothe present disclosure.

At step 100, a streamer is towed in a body of water. At this step inthis process flow, the streamer has an initial orientation. The initialorientation may be a straight line at a particular feather anglerelative to some reference axis, or it may be an approximately straightline at an approximate feather angle, or it may be a non-linearorientation.

At step 102, information relating to crosscurrents in the body of wateris received. As discussed above, this information may be based on forcesproduced by devices along the streamer, or based on configuration data,or based on any source of information related to crosscurrents.

At step 104, a desired streamer orientation is determined, based on thereceived information relating to the crosscurrents. For example, thedesired orientation may be a straight line at a feather angle thatfollows the average crosscurrents in the body of water.

At step 106, the orientation of the streamer is adjusted based on thedetermined desired streamer orientation. This adjustment may be carriedout via positioning devices (e.g. birds) along the length of thestreamer.

FIG. 7 shows another exemplary process flow for an embodiment accordingto the present disclosure.

At step 120, a streamer is towed in a body of water at an initialfeather angle. The streamer may include, among other components, aplurality of deflecting devices arranged at various positions along itslength for providing forces to the streamer.

At step 122, information relating to forces produced by the deflectingdevices along the length of the streamer is received. These forces maybe the forces needed to maintain the streamer at its initial featherangle. This received information may be related to direct or indirectmeasurements of such forces, and it may be based on data received fromthe plurality of deflecting devices.

At step 124, a new desired streamer feather angle is automaticallydetermined. As discussed in more detail above, the desired streamerfeather angle may be determined in order to reduce the amount of forcenecessary from the plurality of deflecting devices, to predictivelyplace the streamer in an advantageous orientation based on measuredcrosscurrent conditions, or in any other way that takes account offorces output by the plurality of deflecting devices. The new desiredstreamer feather angle may be a straight line measured relative to areference axis. The automatic determination may be made without userinput or interaction.

At step 126, the streamer orientation is automatically adjusted based onthe new streamer feather angle. Prior to the automatic adjustment, thecontrol system may or may not require user input and/or confirmation.This adjustment may be carried out via deflecting devices along thelength of the streamer.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A method, comprising: towing a streamer behind avessel in a body of water, wherein the streamer has a presentorientation; a control system receiving information relating tocrosscurrents in the body of water; the control system determining adesired orientation of the streamer based on the received information;and the control system adjusting the present orientation of the streamerbased on the determined desired orientation.
 2. The method of claim 1,wherein the control system receiving information relating tocrosscurrents comprises the control system receiving informationrelating to crosscurrents at a forward portion of the streamer, andwherein the control system determining the desired orientation of thestreamer comprises the control system determining a desired orientationfor a rear portion of the streamer.
 3. The method of claim 1, whereinthe desired streamer orientation is a straight line.
 4. The method ofclaim 3, wherein the straight line is oriented at a determined featherangle, the determined feather angle being measured relative to areference axis.
 5. The method of claim 1, wherein the streamer includesa plurality of geophysical sensors.
 6. The method of claim 1, whereinthe control system is located on the vessel.
 7. The method of claim 1,wherein the control system is located on the streamer.
 8. The method ofclaim 1, wherein the information relating to crosscurrents in the bodyof water is information indicative of direction and speed ofcrosscurrents at a plurality of positions along the streamer.
 9. Amethod, comprising: towing a streamer behind a vessel in a body ofwater, wherein the streamer has a plurality of deflecting devicesarranged therealong, wherein the streamer has a present streamer featherangle that is measured relative to a reference axis; receivinginformation regarding forces exerted by the plurality of deflectingdevices; automatically determining a desired streamer feather anglebased on the received information regarding the forces exerted by theplurality of deflecting devices; and automatically adjusting thestreamer, via the plurality of deflecting devices, to follow thedetermined desired feather angle relative to the reference axis.
 10. Themethod of claim 9, wherein the information regarding the forces exertedby the plurality of deflecting devices comprises information regardingforces exerted by the plurality of deflecting devices on the streamer.11. The method of claim 9, wherein the information regarding the forcesexerted by the plurality of deflecting devices comprises informationregarding forces exerted by the plurality of deflecting devices on thebody of water.
 12. The method of claim 9, wherein the plurality ofdeflecting devices includes a plurality of birds having adjustable wingangles.
 13. The method of claim 9, wherein the reference axis is apreplot direction of the vessel.
 14. The method of claim 9, wherein thereference axis is a front-end direction of the streamer.
 15. The methodof claim 9, wherein the desired feather angle is automaticallydetermined to reduce one or more instantaneous forces exerted by theplurality of deflecting devices, wherein the forces are measuredinstantaneously.
 16. The method of claim 9, wherein the desired featherangle is automatically determined to reduce one or more time-averagedforces exerted by the plurality of deflecting devices.
 17. The method ofclaim 9, wherein the desired feather angle is automatically determinedto maintain one or more instantaneous forces exerted by the plurality ofdeflecting devices below a threshold value.
 18. The method of claim 17,wherein the desired feather angle is automatically determined such thata difference between the desired feather angle and a reference featherangle is minimized.
 19. The method of claim 9, wherein the desiredfeather angle is determined to maintain one or more time-averaged forcesexerted by the plurality of deflecting devices below a threshold value.20. The method of claim 19, wherein the desired feather angle isautomatically determined such that a difference between the desiredfeather angle and a reference feather angle is minimized.
 21. A streamercontrol apparatus, comprising: at least one processor; wherein the atleast one processor is configured to communicate with a plurality ofpositioning devices arranged along a streamer towed behind a vessel in abody of water; wherein the at least one processor is configured todetermine configuration data corresponding to the plurality ofpositioning devices, the configuration data being indicative ofcrosscurrent conditions; and wherein the at least one processor isfurther configured to adjust the plurality of positioning devices basedon the crosscurrent conditions.
 22. The streamer control apparatus ofclaim 21, wherein the configuration data includes a present wing angleof at least one of the plurality of positioning devices.
 23. The methodof claim 21, wherein the at least one processor is configured to adjustthe plurality of positioning devices based on the crosscurrentconditions continuously or periodically.